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Mars takes a star turn in the night sky During this year's close approach to Earth, the Red Planet undergoes a curious reversal — due to orbital mechanics.
Chart shows Mars' path in relation to background stars at nine-day intervals from Sept. 13, 2005, to Jan. 8, 2006, as seen from midnorthern latitudes. The Pleiades star cluster is at left.
Chart shows Mars' path in relation to background stars at nine-day intervals from Sept. 13, 2005, to Jan. 8, 2006, as seen from midnorthern latitudes. The Pleiades star cluster is at / Starry Night
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We're now coming into the home stretch of the last really good apparition of Mars until the summer of 2018.  Now blazing in the late-evening eastern skies like a yellowish-orange lantern, Mars is just over a month away from its closest approach to Earth. 

If you have been following Mars since the start of this year, you'll recall that back then it was shining in the constellation of Scorpius.  At that time it was 208 million miles (333 million kilometers) from Earth.  In contrast, this week Mars' distance from Earth is diminishing to less than 50 million miles (80 million kilometers).

The Red Planet now shines 21 times brighter than it did on New Year's Day.

Also since Jan. 1, Mars has progressed along on an easterly course through the background stars of the Zodiac. It currently resides just inside the western border of the constellation of Taurus the Bull.

Soon that steady eastward course is going to come to a stop.  Actually, for the past few weeks, Mars has appeared to slow in its eastward trajectory, almost seeming to waver, as if it had become uncertain.  Finally, on Oct. 1, it will pause and come to a halt. 

Then, for the next two months, Mars will reverse its course in the heavens and will move backward against the star background — toward the west — almost as if it was going back to retrieve something it lost.  Then, on Dec. 10, it will pause again and resume its normal eastward movement.

Puzzling the ancients
All the planets exhibit this "retrograde motion" at one time or another.  But for the longest time, the ancient astronomers were unable to come up with a satisfactory explanation for it. 

For one thing, while behaving in this strange manner, Mars will also appear to deviate somewhat from its normal course; the retrograde motion will appear to bring it a little below its regular orbital track.  In other words, for those of us watching from Earth, Mars will appear to travel in a loop. 

Yet the Greeks staunchly believed that the sun, moon and planets all moved around Earth in perfect circles.  They had great difficulty in representing and calculating this mysterious loop, and for a long time they had no adequate explanation for it. 

The Greeks finally explained away this anomaly by assuming that the planets moved around the Earth in smaller "epicycles" — that is, a small circle whose center moves along its main orbital circle around Earth, resulting in complex, almost coil-like curves.  Unfortunately, the actual observations of the planets never seemed to fit this strange orbital mechanism, ultimately making the Greeks explanation quite useless.

It was not until the year 1543 — when the great Polish astronomer Nicolaus Copernicus (1473-1543) had his lifelong work "De Revolutionibus" published — that the secret of the odd retrograde loops was finally revealed.  By demoting Earth from its hallowed position at the center of the solar system and replacing it with the sun, he was able to triumphantly explain the riddle of the apparent "backward motion effect" of the planets.

In fact, it's the same effect obtained when passing another car on a highway: both cars are going in the same direction, but one is moving more slowly.  As they pass, the slower car will appear to be moving backward in relation to the faster one. 

Copernicus simply applied the same effect to the planets out in space. 

In the upcoming situation, both Earth and Mars are moving in the same direction around the sun, but the slower one — Mars — appears to move backward compared to the faster one, the Earth.

I would like to stress again that the retrograde motion of Mars — like the apparent motion of the slower car on the highway — is nothing more than an illusion.  The last time Mars underwent retrograde motion (during the summer of 2003), I received inquiries from a number of readers asking if the sun, as seen from Mars, would appear to stop and move backwards across the sky.  The answer is most definitely no.    

The apparent backward motion will manifest itself after Mars arrives at its first stationary point on Oct. 1.  Mars will then begin to loop back toward the west.  Earth will overtake Mars on Nov. 7.  Finally, on December 10, the combined movements of Earth and Mars will cancel out the apparent backward motion, with Mars reaching a second stationary point. 

From then on, Mars will loop back to the east, resuming its normal eastward path among the stars.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for , New York.